The invention relates to the directional solidification of single crystals in vertical bottom seeded crucibles.
The growth of single crystals by directional solidification in vertical bottom seeded crucibles is well established in the art and is referred to as the vertical Bridgman method (Muller, Crystals, Springer-Verlag, New York (1988)).
According to the vertical Bridgman method, crystal growth is accomplished by movement of the crucible or other melt container or by movement of the heat source to create a temperature gradient which results in solidification of the melt. In a variation of a vertical Bridgman method, the vertical gradient freeze method, both the melt container and heater are stationary and heat input is reduced electronically to establish a temperature gradient.
Both the vertical Bridgman method and vertical gradient freeze methods are performed inside tube type furnaces where the melt is heated radially, through the crucible walls, resulting in radial temperature gradients in the growing crystal and the melt. The radial temperature gradients result in unsteady, natural convection in the melt and a curved i.e. non-planar liquid-solid interface. The unsteady convection produces striations i.e. microscopic inhomogeneities, in the grown crystal and convective mixing promotes longitudinal and radial macrosegregation. Furthermore, radial temperature gradients induce thermal stresses and dislocations in the grown crystal. The production of these inhomogeneities and defects compromises the crystalline and chemical perfection of the growing crystal.
Multi-segment tube type furnaces have been constructed to avoid these problems. Such furnaces have been utilized in the successful growth of III-V compounds.